When tensile plastic strain is introduced into steel pipe in the circumferential direction by expansion of the pipe, the yield strength with respect to compressive stress in the circumferential direction due to external pressure (below, the “compressive yield strength”) falls and the pressure at which the steel pipe is crushed by external pressure (below, the “crushing pressure”) falls. The phenomenon arises, as is well known as the “Bauschinger effect”, where after plastic deformation, if applying stress in the opposite direction to the direction in which the plastic deformation was applied, deformation occurs at a lower stress than the original yield strength.
In UOE steel pipe used as line pipe, the pipe is expanded to increase the circularity in the final step. Since tensile plastic strain is introduced in the circumferential direction, there is the problem that the crushing pressure falls. Further, when cold working and using steel plate as well, for example, when applying tensile strain, the compressive yield stress falls and the Bauschinger effect otherwise becomes a problem.
For example, methods using heat treatment to restore the compressive yield strength dropping due to the Bauschinger effect arising due to the cold working strain introduced in the process of production of UOE steel pipe are disclosed in Japanese Patent Publication (A) No. 9-3545 and Japanese Patent Publication (A) No. 9-49025. Japanese Patent Publication (A) No. 9-3545 discloses the method of shaping steel plate by a U-press and O-press into a pipe shape, welding it, expanding it, and heating it to less than 700° C., while Japanese Patent Publication (A) No. 9-49025 discloses the method of further plastically working the pipe by hot working to expand it.
Further, Japanese Patent Publication (A) No. 2004-35925 discloses a method of production of steel pipe enabling recovery of the compressive yield strength falling due to the Bauschinger effect even when reducing the heating temperature to 550° C. or less, particularly 250° C. or less. Further, steel pipes with small occurrence of the Bauschinger effect due to strain introduced at the time of pipemaking and methods of production of the same are disclosed in Japanese Patent Publication (A) No. 9-49050, Japanese Patent Publication (A) No. 10-176239, and Japanese Patent Publication (A) No. 2002-212680.
However, the strain introduced at the time of pipemaking disclosed in these inventions is about 1 to 3% in range or at most 4% or less. The Bauschinger effect on steel plate and steel pipe into which 5% or more of strain is introduced is unclear.
Under the circumstances, in recent years, for example, the technology for use expanded 10 to 30% in oil wells or gas wells (expandable tubular technology) has been developed. The Bauschinger effect on steel plate and steel pipe into which high strain is introduced is therefore becoming an issue. The expandable tubular technology reduces the drilling costs by expanding the oil well steel pipe, which had been inserted into wells and used as is in the past, in the oil wells or gas wells.
Steel pipes able to be used for this expandable tubular technology are disclosed in, for example, Japanese Patent Publication (A) No. 2002-266055, Japanese Patent Publication (A) No. 2002-129283, and Japanese Patent Publication (A) No. 2002-349177. However, these are steel pipes excellent in pipe expandability and crushing strength and corrosion resistance after expansion. Nothing is disclosed about the drop in the crushing strength due to the Bauschinger effect arising due to the introduction of strain envisioning expansion in oil wells.
That is, there have been almost no discoveries regarding the optimum microstructure of steel for suppressing the occurrence of the Bauschinger effect in steel plate into which 5% or more of strain is introduced by cold working or in steel pipe to which 10 to 30% of strain is introduced when expanding oil well pipe inside the oil wells.